1. Field of the Invention
The present invention relates to a communication path quality monitoring method and quality monitoring apparatus, and more particularly to a quality monitoring method and quality monitoring apparatus for monitoring the quality of a communication path passing through a dual-configuration ATM exchange. Generally, an ATM (Asynchronous Transfer Mode) exchange employs a dual configuration for increased reliability. For example, an ATM switch section is constructed in a dual configuration of line 0 and line 1 (working and protection lines), and in the event of a failure of the working line, service is switched to the protection line to provide uninterrupted ATM cell switching operations. In such a case, it is desired to maintain the quality of cell switching so that user cells will not be lost when switching to the protection line is performed.
2. Description of the Related Art
The ATM cell is standardized to 53 bytes in length, with five bytes for the header and 48 bytes for the information field. In a common implementation of an ATM exchange for switching such ATM cells, a routing tag or additional header consisting of one or more bytes is appended to each incoming 53-byte ATM cell and, when output by self-routing, the ATM cell is stripped of the tag or additional header and transmitted out as a 53-byte cell.
As described above, a dual configuration is employed for an ATM exchange for increased reliability. For example, in an ATM exchange having ATM switch sections of line 0 and line 1, if the ATM switch section of line 0 is set as a working (or acting) facility, then the ATM switch section of line 1 is a protection (or standby) facility. Each ATM cell input to a subscriber interface section is delivered simultaneously to the ATM switch sections of line 0 and line 1, and the ATM cell is routed in accordance with the virtual path identifier VPI and virtual channel identifier VCI carried in the header thereof. The ATM cells output from the ATM switch sections of line 0 and line 1 are input to a second subscriber interface section. This subscriber interface section selects the ATM cell from the ATM switch section of line 0 for output, since line 0 is the working facility. If a failure is detected in the ATM switch section of line 0, the subscriber interface section selects the ATM cell from the ATM switch section of line 1 for output. In this way, if a failure occurs in one ATM switch section, cell switching operations can continue without interruption by switching to the other ATM switch section.
In the above-described dual-configuration ATM exchange, when switching operations are being performed using the ATM switch section of line 0 as the working unit, if switching to the ATM switch section of line 1 is performed because of a failure of the working ATM switch section, a minor hardware trouble could result in user cell loss, since the operability of the ATM switch section of line 1 is not verified. Such user cell loss would lead to the degradation of sound or image reproduction quality at the receiving end. Furthermore, in reality it is extremely difficult to locate the user cell loss point in a short time.
It is an object of the present invention to achieve quality monitoring, including the quality relating to the configuration of the protection facility, by using a simple construction.
According to the present invention, there is provided a communication path quality monitoring method comprising the steps of: setting a start bit in a cell passing a first point and having an identifier of a specified communication path; counting the number of cells passing the first point and having the same identifier as the identifier of the cell in which the start bit has been set; detecting the cell having the start bit, at a second point downstream of the first point; counting, after the detection of the cell having the start bit, the number of cells passing the second point and having the same identifier as the identifier of the cell having the start bit; and evaluating the quality of the communication path by comparing the count values obtained at the first and second points.
According to the present invention, there is also provided a communication path quality monitoring apparatus comprising: a setting device for setting a start bit in a cell passing a first point and having an identifier of a specified communication path; a first counter for counting the number of cells passing the first point and having the same identifier as the identifier of the cell in which the start bit has been set; a detector for detecting the cell having the start bit, at a second point downstream of the first point; a second counter for counting, after the detection of the cell having the start bit, the number of cells passing the second point and having the same identifier as the identifier of the cell having the start bit; and means for evaluating the quality of the communication path by comparing the count values of the first and second counters.